In recent years, lithium secondary batteries are widely used as driving power supplies for small electronic devices and the like. Such lithium secondary batteries are mainly constituted of a positive electrode, a nonaqueous electrolytic solution and a negative electrode, and in particular, in which a lithium compound oxide such as LiCoO2 is used as the positive electrode and a carbon material or lithium metal as the negative electrode. The nonaqueous electrolytic solution for use in such lithium secondary battery includes carbonates such as ethylene carbonate (EC) and propylene carbonate (PC).
However, secondary batteries having better characteristics, i.e., the battery cycling property and electrical capacity are needed.
In a lithium secondary battery in which LiCoO2, LiMn2O4, LiNiO2 or the like is used as the positive electrode, part of a solvent in a nonaqueous electrolytic solution locally decomposes oxidatively during charging, and the decomposition products inhibit a desirable electrochemical reaction of the battery, thereby lowering the battery performance. This is thought to result from the electrochemical oxidation of the solvent at the interface between the positive electrode material and the nonaqueous electrolytic solution.
Moreover, in another lithium secondary battery using, highly crystallized carbon materials such as natural graphite and artificial graphite as the negative electrode, a solvent in a nonaqueous electrolytic solution reductively decomposes on the surface of the negative electrode during charging. Therefore, part of ethylene carbonate (EC) which is generally and widely used as a nonaqueous electrolytic solution solvent is also reductively decomposed through repeated charging and discharging, thereby lowering the battery performance.
As substances which improve the battery characteristics of this lithium secondary battery, patent document 1 discloses ethyleneglycol diacetate; patent document 2 discloses acetates such as butyl acetate and diacetates such as ethylene diacetate; patent document 3 discloses vinyl acetate; and patent document 4 discloses allyl acetate. These documents suggest improved cycle life. Moreover, patent document 5 discloses a secondary battery with increased oxidation resistance and improved charge/discharge capacity by optimizing the concentration of a lithium salt by using aliphatic esters such as methyl formate. However, in order to increase the capacities of lithium secondary batteries, nonaqueous electrolytic solutions and lithium secondary batteries having even better cycling properties and electrical capacities are needed.
Patent document 1: Japanese Unexamined Patent Publication No. H07-272756
Patent document 2: Japanese Unexamined Patent Publication No. H09-97626
Patent document 3: Japanese Unexamined Patent Publication No. H11-273724
Patent document 4: Japanese Unexamined Patent Publication No. H11-273725
Patent document 5: Japanese Unexamined Patent Publication No. H09-306538